mgmt/image_header: Update dependency

Package depended on boot/bootuil from mcuboot now
it depends on boot/stub.

The only thing that was needed was definition of
structure image_header that is present in both of them.

Not using boot/bootutil can reduce compile time.

Signed-off-by: Jerzy Kasenberg <jerzy.kasenberg@codecoup.pl>

mgmt/image_header/pkg.yml

@@ -27,7 +27,7 @@ pkg.homepage: "http://mynewt.apache.org/"
 pkg.keywords:
 
 pkg.deps:
- - "@mcuboot/boot/bootutil"
+ - "@apache-mynewt-core/boot/stub"
 
 pkg.lflags:
  - "-uimage_header"